As generally known in the art, a cylindrical lithium rechargeable battery includes a cylindrical electrode assembly having a center pin, a cylindrical can to which the electrode assembly is coupled, and a cap assembly coupled to the top opening of the can for sealing the can.
Such lithium rechargeable batteries generally have capacities ranging from 2000 to 2400 mA, and have been widely used for notebook PCs, digital cameras, camcorders, and the like, which require electric power with high capacity. Additionally, such rechargeable batteries are connected in series or in parallel to provide a desired voltage or capacity, and take the form of battery packs having predetermined shapes and including safety devices.
In one conventional cylindrical lithium rechargeable battery, an anode plate coated with an anode active material, a separator, and a cathode plate coated with a cathode active material are stacked and one end of the stacked structure is coupled to a bar-shaped winding axis. The structure is then wound into the shape of a cylinder to form an electrode assembly. Next, a center pin is coupled to the electrode assembly and the resultant structure is inserted into a cylindrical can. Then, an electrolyte is injected into the cylindrical can and a cap assembly is coupled to the top of the cylindrical can to complete a substantially cylindrical bare cell.
The cylindrical lithium secondary battery described above also includes a safety device for preventing the battery from exploding upon overcharge. The safety device includes a safety vent capable of deformation upon increases in the internal pressure of the battery due to overcharge. The battery also includes a current interruption device (CID) that interrupts the circuit through the substrate which is deformed due to the deformation of the safety vent. In addition, the battery includes a secondary protection device that interrupts the circuit upon increases in temperature.
Such safety devices operate as follows. When a lithium rechargeable battery is in an overcharged state, the electrolyte in the battery evaporates from the top region of the electrode assembly, thereby increasing electric resistance. Additionally, the central portion of the electrode assembly deforms, resulting in precipitation of lithium. Further, the battery undergoes a rapid increase in temperature because the increased electric resistance at the top region of the electrode assembly causes local heat emission.
Under these circumstances, the internal pressure of the battery rapidly increases due to the additives in the electrolyte, such as cyclohexyl benzene (CHB) and biphenyl (BP). Such additives decompose upon overcharge and generate gas. The increased internal pressure pushes the safety vent (which is part of the cap assembly) out of its place, causing the current interruption device (CID) disposed over the safety device to break, which results in interruption of the electric current. In other words, the printed circuit pattern formed on the CID is broken, thereby interrupting the electric current. Current interruption results in termination of the overcharged state, thereby making it possible to prevent the battery from exploding and igniting. Also, if the internal pressure of the battery is greater than the critical pressure due to overcharge, the safety vent may break, discharging gas to the exterior of the bare cell through an opening in the upper cap plate.
The center pin may be made cylindrical in shape by winding a metal plate. Alternatively, the center pin may originally be formed into an integral cylinder. When the center pin comprises a wound metal plate, the metal plate includes a slit where both ends of the metal plate contact each other. Thus, an electrolyte or gas may be transferred through the slit. When the center pin is an integral cylinder, there is a void volume inside the center pin. The void volume causes delay in the deformation or breakage of the safety vent. The void volume must be reduced in initial overcharge to allow the safety vent to operate promptly and to permit the center pin to serve as a gas discharge path when the internal temperature of the battery reaches 80 to 250° C.